Abstract

A calibrated Fourier transform spectrometer, known as the High-Resolution Interferometer Sounder (HIS), has been flown on the NASA U-2 research aircraft to measure the infrared emission spectrum of the earth. The primary use—atmospheric temperature and humidity sounding—requires high radiometric precision and accuracy (of the order of 0.1 and 1°C, respectively). To meet these requirements, the HIS instrument performs inflight radiometric calibration, using observations of hot and cold blackbody reference sources as the basis for two-point calibrations at each wavenumber. Initially, laboratory tests revealed a calibration problem with brightness temperature errors as large as 15°C between 600 and 900 cm−1. The symptom of the problem, which occurred in one of the three spectral bands of HIS, was a source-dependent phase response. Minor changes to the calibration equations completely eliminated the anomalous errors. The new analysis properly accounts for the situation in which the phase response for radiance from the instrument itself differs from that for radiance from an external source. The mechanism responsible for the dual phase response of the HIS instrument is identified as emission from the interferometer beam splitter.

Figures (9)

Functional schematic of HIS optics. The primary mirror, collimating mirror, and focusing mirrors are shown as lenses to simplify the drawing. Plane reflecting surfaces are shown as dashed lines, and the two paths of the interferometer are functionally represented. The complete instrument is ~2.7 m (9 ft) long and fits into the 45.7-cm (18-in.) diam wing pods of the NASA U-2 aircraft.

Uncalibrated magnitude (a) and phase (b) spectra of black-body sources for HIS spectral band I. The curves are labeled by the temperature of the source. The phase labeled 300K–77K Difference is the phase of the difference of the 300 and the 77 K spectra. The magnitude spectra are shaped by a Gaussian numerical filter and display CO2 absorption features and channeling as discussed in the text. Note the substantial differences among the phase responses.

Uncalibrated magnitude (a) and phase (b) spectra of black-body sources for HIS spectral band II. The curves are labeled as in Fig. 2. Here also the magnitude spectra are shaped by a numerical filter and display channeling as discussed in the text. The very deep and numerous lines in the magnitude spectra are due to H2O absorption. In contrast to band I, the phase spectra are very similar and quite linear.

Brightness temperature spectra for band I. The spectrum with the large deviations from the measured 280.2 K temperature of the blackbody source is derived from the original calibration analysis using magnitude spectra. The spectrum which accurately reproduces a constant brightness temperature is from the modified analysis presented here. The large variance between 600 and 650 cm−1 is caused by the low instrument transmittance in that region. Changes have since been made to improve the throughput in this region.

Brightness temperature spectra for band II (a) and band III (b). The calibration for these bands is good and essentially identical for the standard and modified analyses. The larger variance beyond 1400 cm−1 for band II and centered at 2350 cm−1 for band III are due to H2O and CO2 in the path of the interferometer.

Transmittance of a beam splitter of the same construction as the HIS instrument beam splitter. The absorption feature centered at ~740 cm−1 is responsible for the dual phase response of the instrument. The sinusoidal fit to the transmittance is used to approximate the reflectance for beam splitter emittance and efficiency calculations.

Simplified beam splitter models for emittance and efficiency calculations. The expressions in terms of the emittance ∊ and reflectance ρ represent the amplitudes of the beam at various locations. The rays on the left represent the emission process, and those on the right represent the passage of an external beam.

Beam splitter emittance and efficiency estimates labeled by the model assumed. Model 2′ is the same as model 2 with the emitting and reflecting surfaces reversed in order. The dashed emittance curves are from the data of Fig. 6, and the solid curves are from measurements with the HIS instrument.

Comparison of instrument responsivity calculated from optical component transmittances and detector responsivity to the end-to-end responsivity from calibration measurements. The beam splitter efficiencies used in the calculated responsivities are those deduced from measurements with the HIS interferometer itself. The calculations using model 2 seem to account properly for actual beam splitter efficiencies.